Standards for AN determination in petroleum oils are based on the acid-base titration techniques in non-aqueous systems (American National Standards D3339-87; International Electrotechnical Commission Standard, 296, 1982). These techniques are time and labour consuming, and difficult for automation. There are used toxic and flammable solvents, for example toluene, as well as non-aqueous alkali titrants which are very sensitive to carbon dioxide contamination from the atmosphere. Difficulties are increasing at the determination of AN.ltoreq.0.03 mg KOH/g oil (Transformer, White, Basic and other oils). In this case the titration should be carried out in a nitrogen atmosphere and besides the indicator, the colour witness (cobalt nitrate solution) is used also for more correct determination of the end point of the titration (American National Standard D3339-87; International Electrotechnical Commission Standard 296, 1982).
The pH-metric technique developed earlier by us for determination of acid value in vegetable oils and corresponding reagent-precursor (0.20 M triethanolamine in a mixture 1+1 of water and isopropanol, (vol.); Ya. I. Tur'yan, O. Berezin, I. Kuselman and A. Shenhar, Israeli Pat. Appl. 110192, 01.07.94) are free from indicated drawbacks. However, its limit of quantitation is 0.03 mg KOH/g oil. It is sufficient for vegetable oils but it does not satisfy requirements of the petroleum oil analysis.
Another reagent (0.01 M NaOAc in water: Ya. I. Tur'yan, V. F. Pokhodzei and D. A. Krukier, Izv. Vyssh. Uchebn. Zaved., Pisshch. Technol., Krasnodar, USSR, No. 3, 1989, p. 39) was developed for pH-metric acid value determination in aqueous samples. This reagent allowed to decrease the limit of quantitation, but is not suitable for AN determination in oils since acid from oils cannot be extracted completely into an aqueous reagent. The aim of the present invention is to provide a reagent for the pH-metric AN determination in petroleum oils in the range AN.ltoreq.0.03 mg KOH/g oil.